1. Field of the Invention
The present invention is directed to a method and circuit suitable for use in pacemakers, cardioverters, defibrillators, and the like, making use of a lead having a tip with multiple electrodes to detect cardiac rhythm abnormalities, such as fibrillation and tachycardia.
2. Description of the Prior Art
A cardiac lead typically has a proximal end with a connector adapted for electrical and mechanical connection to a cardiac-assist device, such as a pacemaker, cardioverter or defibrillator, and an opposite distal end, at which one or more electrodes is/are located. Between the distal end and the proximal end, the lead has a flexible insulating sheath or jacket, containing conductors depending on the number of electrodes.
The electrodes are exposed conductive surfaces at the distal end of the lead. Conventional electrode configurations include a unipolar configuration and a bipolar configuration. In a unipolar configuration, there is only one electrode at the distal end, typically a hemisphere covering the distal tip. Typically the housing, or a portion thereof, of the cardiac assist device is used as the indifferent or return electrode. A bipolar lead has two electrode surfaces, separated from each other by a spacing. Typically one of these electrodes is formed as a hemispherical electrode at the distal tip of the lead, and the other is a ring electrode, which annularly surrounds the sheath, located a short distance behind the tip electrode.
In most modern cardiac assist devices, the electrode lead is not only used to deliver an appropriate cardiac assist regimen in the form of electrical pulses, but also is used to detect cardiac activity. The detection of cardiac activity can serve many purposes, such as for use in determining whether adjustments to the cardiac assist regimen are necessary, as well as for identifying cardiac rhythm abnormalities which may require immediate preventative action, such as the occurrence of tachycardia or fibrillation. Particularly in the case of a cardioverter or a defibrillator, which is normally passive unless and until tachycardia or fibrillation is detected, it is important not only to reliably detect tachycardia or fibrillation when they occur, but also it is important not to misidentify a non-emergency cardiac rhythm abnormality as tachycardia or fibrillation, since administering the emergency regimen to a healthy heart can possibly create an emergency situation where none exists. Moreover, at least in the case of a defibrillator, unnecessary triggering of the extremely strong defibrillation energy will cause considerable discomfort to the patient.
An electrode lead for a cardiac pacemaker is disclosed in U.S. Pat. No. 5,306,292 which has a distal tip with a number of closely spaced electrodes thereon, with the remainder of the hemispherical surface of the distal tip of the electrode being non-conducting. Circuitry in the pacemaker housing, connected to the respective electrodes via the electrode lead cable, allows the total conductive area and geometry of the distal tip of the electrode lead to be selectively varied, by activating the electrodes in different combinations. For example, the combination of electrodes (i.e. conductive surfaces) at the electrode tip which provides the lowest stimulation threshold can be determined by an autocapture unit, so that energy consumption can be reduced.
Many algorithms are known for analyzing the detected signal wave forms obtained with unipolar and bipolar leads. A prerequisite to the proper functioning of most of these algorithms is that the signal which enters into the algorithm be relatively noise-free. The detected signal, in its raw form, can be corrupted by noise produced by electromagnetic interference in the patient""s environment, as well as by muscle activity. Such noise may mimic a fibrillation pattern, for example, particularly in the case of a unipolar lead, but also to a certain extent with a bipolar lead.
Conventional noise-removing techniques involve filtering and other types of signal editing procedures.
After making the incoming signal reasonably noise-free, conventional detection algorithms analyze the signal by undertaking one or more threshold comparisons and/or by analyzing the rate of occurrence of a particular characteristic of the signal (i.e., maxima, minima, zero crossings, etc.) over a given period of time. Comparison of the signal waveform to stored signal templates, respectively representing previously-obtained abnormal signals, is also a known technique. In this manner, a determination is made as to whether the incoming signal represents normal sinus rhythm, a PVC, tachycardia, atrial fibrillation, ventricular fibrillation, etc.
It is an object of the present invention to provide a method and a circuit for analyzing signals obtained with such a cardiac lead for the purpose of detecting cardiac abnormalities so that remedial action can be taken.
The above object is achieved in accordance with the principles of the present invention in a method and circuit for use with a cardiac lead having multiple electrodes or conductive surfaces (xe2x80x9cdotsxe2x80x9d) at its distal tip, wherein each dot has its own conductor in the lead, in which a unipolar signal from that dot is conducted. A differential signal is obtained between the respective unipolar signals for two such dots. It has been found that the respective unipolar signals of a lead with electrodes (conductive surfaces) very close to each other, on the order of a few tenths of a millimeter of separation, exhibit morphologic differences to a greater extent during fibrillation than during normal heart activity. This morphologic difference is exploited in the method and circuit of the present invention, wherein appropriate editing of the differential signal is undertaken to detect these morphologic differences. In the method and circuit of the invention, the differential signal is supplied to a bandpass filter, which can be formed from a low-pass filter having a corner frequency of approximately 10 Hz followed by a 5 Hz high-pass filter. The signal from this bandpass filter is then rectified and then supplied to a low-pass filter, preferably a 0.5 Hz low-pass filter. The output from this low-pass filter is supplied to a threshold detector. Signals above the threshold of the threshold detector represent detection of a signal representing cardiac abnormality. Signals above the threshold with this type of signal editing have been found to occur when a state of fibrillation exists. By similarly analyzing a number of differential signals respectively obtained from different pairs of electrode dots, the reliability of the fibrillation detection can be enhanced even further. Preferably, a lead having a tip with two to seven dots is employed.
The dot-like electrodes of the lead used with the inventive method and circuit are individually formed of conductive material, and are separated at the surface of the distal tip of the electrode by electrically insulating material. The arrangement of the electrode dots can include a centrally disposed electrode dot, with a number of further of electrode dots annularly arranged around the centrally disposed dot. The annularly arranged electrode dots can be located at radially symmetrical positions relative to the centrally disposed dot.
A very small distance between two dots results in the cardiac differential signal detected by a pair of dots having a slightly reduced amplitude in comparison to the signal detected by a conventional unipolar or bipolar lead, however, the signal detected by each electrode dot pair already has a significantly reduced noise content in comparison to conventionally obtained signals, so that much less editing in order to remove noise artifacts from the signals is necessary.
Each electrode dot preferably has a diameter of 0.5 mm, with the edge-to-edge distances among all of the respective dots being approximately equal.
In accordance with the invention, one appropriate method for analyzing the signals obtained from the respective electrode dots is to obtain unipolar signals from the respective dots with the cardiac assist housing serving as the ground level. By producing a difference signal between respective signals from two dots, a bipolar differential signal is obtained, although this will be different from a conventional bipolar signal obtained with a tip electrode and a ring electrode configuration. Multiple difference signals are thus available for analysis. It is also possible to employ one of the electrode dots as a ground reference, and to refer all of the difference signals to the signal obtained from that one dot.